Capacitor electrolyte

ABSTRACT

A capacitor for an implantable medical device is presented. The capacitor includes an anode, a cathode, a separator therebetween, and an electrolyte over the anode, cathode, and separator. The electrolyte includes ingredients comprising acetic acid, ammonium acetate, phosphoric acid, and tetaethylene glycol dimethyl ether. The capacitor has an operating voltage ninety percent or greater of its formation voltage.

CROSS REFERENCE AND INCORPORATION BY REFERENCE

This patent disclosure claims the benefit of U.S. provisional patentapplication Ser. No. 60/695,670 filed on Jun. 30, 2005, entitled,“CAPACITOR ELECTROLYTE,” the entire contents of which, includingexhibits appended thereto, are incorporated herein by reference.Attention is directed to patent application Ser. No. 10/977,584 filed onOct. 29, 2004 entitled, “PROCESSES AND SYSTEMS FOR FORMATION OF HIGHVOLTAGE, ANODIC OXIDE ON A VALVE METAL ANODE” published as2006-0091020-A1 on May 4, 2006 and patent application Ser. No.10/832,766 entitled, “ELECTROLYTE FOR ELECTROLYTIC CAPACITOR,” now U.S.Pat. No. 7,081,141 the contents of which, including exhibits appendedthereto, are incorporated herein by reference in their entireties.

FIELD OF THE INVENTION

The present invention generally relates to implantable medical devicesand, more particularly, to capacitors.

BACKGROUND OF THE INVENTION

Implantable medical devices (IMDs) detect and deliver therapy for avariety of medical conditions in patients. Exemplary IMDs includeimplantable pulse generators (IPGs) or implantablecardioverter-defibrillators (ICDs). ICDs include a circuit that detectsabnormal heart rhythms and automatically delivers therapy to restorenormal heart function. An ICD circuit includes, inter alia, a battery, acapacitor, and a control module. The battery supplies power to thecontrol module and the capacitor. The control module controls electricaltherapy delivered to a patient. For example, the control module signalsa switch, coupled to the capacitor, to open or close, which controlswhether energy is released by the capacitor. The capacitor deliversbursts of electric current through a lead that extend from the ICD tomyocardial tissue of the patient.

Electrolytic capacitors (e.g. tantalum, aluminum etc.) are typicallyused since these capacitors attain high energy density in a low volumepackage. Generally, a tantalum electrolytic capacitor's formationvoltage is typically three to four times the capacitor's rated voltage.See, John Gill, Basic Tanatlum Capacitor Technology, AVX Journal, p.3.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description and the accompanying drawings, wherein:

FIG. 1 a is a cross-sectional view of an electrolytic capacitor;

FIG. 1 b is a cross-sectional view of an implantablecardioverter-defibrillator including an electrolyte capacitor;

FIG. 2 is a graph that depicts exemplary electrolyte voltage regions;

FIG. 3 is a graph that depicts leakage current trends versus voltage;

FIG. 4 is a graph that depicts energy input versus voltage;

FIG. 5 is a graph that depicts output energy trends versus voltage;

FIG. 6 is a graph that depicts efficiency trends versus voltage;

FIG. 7 is a graph that depicts delivered energy versus equivalent seriesresistance (ESR);

FIG. 8 is a graph that depicts capacitance versus voltage;

FIG. 9 is a graph that depicts delivered energy trends versus appliedvoltage;

FIG. 10 is a graph that depicts efficiency trends versus appliedvoltage;

FIG. 11 is a graph that depicts efficiency trends versus applied voltagefor discharge to 25% of the voltage.

FIG. 12 is a cross-sectional view of an electrolytic capacitor;

FIG. 13 is a flow diagram for producing a capacitor capable of anoperating voltage that is 90% or more of its formation voltage.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description of the preferred embodiment is merelyexemplary in nature and is in no way intended to limit the invention,its application, or uses. For purposes of clarity, similar referencenumbers are used in the drawings to identify similar elements.

One embodiment of the present invention is directed to an electrolyticcapacitor that exhibits an operating voltage of 90% or more of theformation voltage. Operating voltage is the maximum voltage to which acapacitor is charged. Formation voltage is a voltage maintained during aprocess in which an oxide layer, an insulator, is grown on a surface ofthe anode. Operationally, formation voltage is the voltage above whichthe anode begins to draw significantly higher current as substantialamounts of oxide are grown across the surface of the anode to supportthe higher voltages. Formation voltage and formation temperature followthe relationship T_(f)V_(f)=constant, where T_(f) is the formationtemperature and V_(f) is the formation voltage. [A F Torrisi, “Relationof Color to Certain Charatcteristics of Anodic Tantalum Films,” Journalof the Electrochemical Society, Vol 102, No 4, pp176-180, April 1955.]For oxides formed at different temperatures, the formation voltageshould be corrected to the application temperature for appropriatecomparison to the operating voltage.

In another embodiment, operating voltage of 90% or more of the formationvoltage is based upon the electrolyte of the electrolytic capacitor.More specifically, the present invention's rated voltage is about 98% ofthe formation voltage. An exemplary electrolyte includes ingredientscomprising acetic acid, ammonium acetate, phosphoric acid, tetraethyleneglycol dimethyl ether, and water.

Another aspect of the present invention relates to designing anelectrolytic capacitor capable of achieving an operating voltage of 90%or more of the formation voltage. There are several designconsiderations. For example, one consideration is dielectric breakdownof an electrolytic capacitor. Another exemplary consideration isequivalent series resistance (ESR) related to movement of charge betweenthe anode and the cathode of the electrolytic capacitor. Skilledartisans will appreciate that the present invention achieves 15-18%improvement in energy density for the electrolytic capacitor.

FIG. 1 a is a cross-sectional view of an electrolytic capacitor 10.Electrolytic capacitor 10 includes an anode 11, a cathode 18, aseparator 16 therebetween, and an electrolyte 17. Anode 11 includes ametal layer 12 and an oxide layer 15. Metal layer 12 is an anodicmaterial such as tantalum (Ta), or other suitable material. Oxide layer15 forms in pores 14 on metal layer 12. Formation of oxide layer 15occurs when metal layer 12, exposed to a different electrolyte in abath, has high voltage (e.g. up to 390 volts (V)) applied between metallayer 12, which is the positive terminal and a negative terminal (notshown). FIG. 1 b is a cross-sectional view of an implantablecardioverter-defibrillator 1 including the electrolytic capacitor 10.

One embodiment of the invention relates to an electrolyte that includesglacial acetic acid (HC₂H₃O₂), ammonium acetate (NH₄(C₂H₃O₂)),phosphoric acid (H₃PO₄), water, and tetraethylene glycol dimethyl ether(CH₂(OCH₂CH₂)₄OCH₃). Exemplary quantities for the electrolyteingredients are presented in Tables I and II below. These electrolyte 17formulations were evaluated after being introduced to a set of 0.095inch thick anode slug in electrolytic capacitors. Electrolyte 17formulations were selected based on differing conductivities and/orbreakdown voltages.

As shown in Table I, each electrolyte formula included a variety ofdifferent molarities for each ingredient. For example, tetraethyleneglycol dimethyl ether ranged from about 0.9 molarity for electrolyte 1to about 1.43 molarity for electrolyte 4. Water is the remainingportion.

TABLE I Molarities of ingredients to form the electrolyte Exemplaryquantities Glacial Tetraethylene of ingredients for acetic AmmoniumPhosphoric glycol dimethyl electrolyte acid Acetate acid ether 1 2.5 2.00.03 0.9 2 2.1 1.65 0.04 1.25 3 1.7 1.3 0.05 1.6 4 1.9 1.48 0.045 1.43 52.02 1.58 0.042 1.32

TABLE II Mass (grams) per 2-liters of solution Glacial TetraethyleneElec- acetic Ammonium Phosphoric glycol dimethyl trolyte acid Acetateacid ether Water 6 300 308 6 400 1060 7 252 254 9 556 1010 8 243 244 10587 998

In one embodiment, the best electrolyte formulations depend on thedesired operating voltage of capacitor 10. For example, electrolyte 8 inTable II is the best electrolyte 17 formula for an operating voltage of255V.

Turning now to FIG. 2, a voltage curve from an electrolyte voltage testis shown which allows a designer of capacitor 10 to visually assess theperformance of electrolyte 17 across a range of voltages. In this test,a constant current is applied to a tantalum wire in a sample ofelectrolyte. The voltage is monitored as a function of time. Theobserved voltage typically rises linearly for some time, then risessomewhat more slowly with increasing noise as parasitic processes absorbmore of the applied current.

The voltage of the anode immersed in electrolyte 17 has an initiallinear rise (Region A), a transition region (Region B) and a relativelyflat, noisy region characterized by continuous steady-state breakdownand repair (Region C). Electrolytic capacitor 10 is least likely todegrade over time while operating in the linear region (Region A). Thehighest voltage in this region is a nominal working voltage 20 ornominal electrolyte voltage. Upon increasing voltage into the transitionregion (Region B), a trade-off occurs between the efficiencies of ahigher operating voltage and charge time stability of capacitor 10. Inthe transition region, charge time begins to show some instability dueto greater oxide susceptibility to reaction with electrolyte 17. Beyondthe transition region, actual breakdown occurs of oxide layer 15.

Numerous factors may be contemplated to design capacitor 10. In oneembodiment, capacitor 10 is designed by consideration of the workingvoltage and the nominal working voltage. Nominal working voltagemeasures the ability of electrolyte 17 to withstand voltage whereasworking voltage is the capacitor-level assessment of the highest voltageat which electrolyte 17 may be used. In one embodiment, a designer ofcapacitor 10 may set working voltage above the nominal working voltageat a value that balances competing design goals. To illustrate, bysetting working voltage at a level above the nominal working voltage,battery size of an implantable medical device (IMD) may increase due toincreased charge time and leakage losses but ESR-type heating losses maybe reduced. Reduction of ESR is due to a more conductive electrolyte 17,which also reduces the size of capacitor 10. Other design considerationsmay include the lifetime of capacitor 10, operating temperature ofcapacitor 10 charge time of capacitor 10, battery life, overall IMDsize, and operating voltage of capacitor 10. Electrolyte 17 can be used50 volts (V) higher than the nominal electrolyte voltage. Additionally,electrolyte 17 achieved an operating voltage that is above 90% of anodeslug formation voltage. In one embodiment, capacitor 10 has an operatingvoltage that is above 93% of anode slug formation voltage. In anotherembodiment, capacitor 10 has an operating voltage that is above 95% ofanode slug formation voltage. In still yet another embodiment, capacitor10 has an operating voltage that is above 98% of anode slug formationvoltage.

Operating at a higher fraction of formation voltage increases energydensity, since E=(½) CV²where E is energy, C is capacitance, and V isoperating voltage of capacitor 10. Capacitance is defined as C=(KA)/dwhere K is the dielectric constant, A is the surface area of oxide layer15, and d is the thickness of oxide layer 15.

Capacitance decreases and operating voltage increases when oxide layer15 is formed to a higher voltage (and consequently to a greaterthickness). The increase in energy density from these factors mayoutweigh possible limitations associated with lower conductivity (higherrequired porosity, lower powder density, thinner anodes, and/or reducedhigh frequency performance, etc.), increased leakage current, or chargetime. Increased energy density, which depends on the increased operatingvoltage, reduces the size of the ICD.

In order to further enhance the performance of electrolyte 17, additivesmay be added to the mixture of electrolyte 17 ingredients listed inTables I and II. Organic solvents or salts (e.g. ammonium azelate,potassium phosphate etc.) may be used to adjust (increase or decrease)the nominal working voltage while reducing the conductivity associatedwith electrolyte 17. For electrolyte not at a pH 7.0, increasedconductivity may be achieved by addition of hydronium ions, hydroxideions, or other suitable additive(s).

In another embodiment, improving characteristics associated with oxidelayer 15 may also improve the operating voltage. Oxide susceptibility toleakage and breakdown may be significantly reduced, allowing operationto occur beyond the nominal working voltage. The selection ofingredients for electrolyte 17 may synergistically enhance capacitor 10(e.g. the higher voltage capacitor, a desirable therapy voltage with athree-capacitor system, higher energy density capacitor etc.) incombination with suitably processed capacitor oxides.

Oxide properties and thickness are affected by oxide formationconditions. Exemplary oxide formation conditions include temperature ofmetal layer 12 in an electrolytic environment (e.g. electrolyte bathetc.), hold time at final formation voltage, and the final electricalcurrent density through metal layer 12 at termination of the formationprocess.

Experimental data from two separate experiments confirmed thatelectrolyte 17 achieves an operating voltage of 90% or more of theformation voltage. The two experiments also explored the manner in whichcompeting design goals were affected. As a preliminary manner,electrolytes 1 through 8 in Tables I and 11 were formulated to providenominal electrolyte voltages between 175 V and 225 V for bothexperiments.

FIGS. 3-5 relate to the first experiment whereas FIGS. 7-12 relate tothe second experiment. Several general observations were made based uponthe experiments. For example, capacitor 10 does not become abruptlynonfunctional at a higher voltage. Electrolyte 17 is usable for somewhathigher voltages, with some possibility of increasing charge times overmany charging cycles. The primary trade-off is between leakage currentand ESR. These properties may change over the life of capacitor 10.

Increasing conductivity of electrolyte 17 increases the accessibility ofcharge stored in the interior of the anode slug. Electrolyte 17 shouldbe sufficiently conductive to allow access to charge in the interior ofthe anode slug on the timescale of the application charge/dischargecycle.

Referring to Table 3, nine capacitors were examined to determine theeffects of a particular electrolyte 17 formula from Table I.

TABLE III Capacitor inventory Nominal Electrolyte ElectrolyteResistivity Ohm Capacitance Electrolyte Voltage Centimeters Anode SerialmicroFarad ESR Example (Volts) (Ω cm) Number Number (μF) (ohm (Ω)) 1 17523.3 4 35 344.0 1.081 2 200 34.7 2 38 335.9 1.469 2 200 34.7 5 23 329.21.529 2 200 34.7 8 5 338.6 1.412 3 225 42.0 3 6 327.4 2.024 3 225 42.0 621 329.9 2.061 3 225 42.0 7 46 325.5 2.041All capacitors 10 were constructed from slugs prepared under the sameformation conditions and formed to 225 V. Conservative working voltageswere used for formulations of electrolyte 17. Typical formationconditions and/or processes are disclosed in U.S. Pat. No. 5,716,511entitled “Anodizing electrolyte and its use and U.S. Pat. No. 6,480,371entitled. “Alkanolamine-phosphoric acid anodizing electrolyte” both ofwhich are incorporated by reference in relevant parts. Low leakagecurrent powders with CV in the range of 10,000 to 20,000microfarad*volt/gram (uF V/g) are appropriate for this testing, where CVrepresents a standard rating of a capacitor powder, the expectedcapacitance multiplied by the voltage. Suitable powders are availablefrom H C Starck ated or Cabot Corporation.

All exemplary electrolytes 17 show a clear progression in capacitanceand ESR. As the electrolyte resistivity increases, average ESRprogresses from 1.1 Ω to 1.5 Ω to 2.0 Ω nearly doubling as resistivityis doubled. Additionally, capacitance drops from 344 μF to 335 μF to 328μF, reflecting the reduced electrical access to the slug interior at 120Hz, which represents the relevant time scale of ICD function.

FIG. 3 compares average leakage currents for the three electrolytes ofTable III. On FIG. 3, traces are labeled relative to the nominalelectrolyte voltage of capacitor 10. Leakage current rises rapidly uponapproaching the formation voltage. The lower nominal voltage electrolytemay exhibit higher leakage current; however, the small sample size lackssufficient statistical data. A tradeoff between leakage current andESR/capacitance may exist. Leakage currents are measured after fiveminutes at a particular voltage, according to standard practices.Results are averaged over the available parts. Capacitance, ESR andleakage current all contribute to energy density. Energy density is asignificant consideration for an ICD. The present invention achieves15-18% improvement in energy density for the electrolytic capacitor.

FIG. 4 shows the variations in input energy with charge voltage ofelectrolyte 17. The dashed line is a projection of the 125 V energy tohigher voltages, assuming an ideal scale with V²of E=(½) CV² Forcapacitor 10, about a joule of additional energy is required at 225 Vdue to energy losses. Electrolytes 1-3 are tabs 101, 102, and 103,respectively.

FIG. 5 represents the output energy trends for capacitor 10 dischargedinto a 16.7Ω load, which generally correlates to a nominal 50 Ω patientload for a three capacitor system in an IMD. These output energies aremeasured on discharging to 0 volts. The lower nominal voltage, higherconductivity electrolytes have consistently higher output energies. At225 V, there is a 0.5 joules(J) difference between the electrolyte 1 andelectrolyte 3. Within this data set, electrolyte 1 for capacitor 10delivers the most energy. It was also observed that ESR losses outweighleakage current effects.

FIG. 6 depicts the general downward slope in efficiency primarilyrelated to charging losses. The gap between the curves is primarilyrelated to ESR differences.

FIG. 7 depicts delivered energy for each of capacitors 10 charged to 225V as a function of ESR. As shown, data below 1.3 Ω representselectrolyte 1; between 1.3 Ω and 1.8 Ω represents electrolyte 2; andelectrolyte 3 capacitors appear above 1.8 Ω. As expected, there is areduction in delivered as the internal resistance of capacitor 10increases, and the reduction is more pronounced with smaller loads. Thiseffect is due to the ESR (measured at 120 Hz) acting as a voltagedivider in series with the load resistance.

From the first experiment, a number of observations were made. Forexample, electrolyte 17 is effectively used at 50 V above its nominalbreakdown voltages. Moreover, leakage current losses do not compete withESR losses. Additionally, conductivity outweighs leakage current inchoosing an electrolyte formulation. The impact of slower ionic accessto the interior of the slug was assessed to be less than 2% of deliveredenergy efficiency.

For the second experiment, capacitors 10 were used to examine thedependence of delivered energy and charge/discharge efficiency onelectrolyte conductivity. Capacitors were made of two types of anodepowder, Type 1 and Type 2. Four different electrolytes of increasingconductivity and decreasing nominal working voltage were used.

Capacitors were tested at various voltages, as shown on Table IV.

TABLE IV Test voltages for capacitors by electrolyte Anode Powder 1^(st)test 2^(nd) test 3^(rd) test 4^(th) test 5^(th) test Type voltagevoltage voltage voltage voltage Type 1 150 175 200 220 Type 2 150 175200 220 260

At each voltage, a series of tests were performed including 5 minuteleakage current test, ESR, and capacitance, charge energy, charge time,and discharge energy. For the first two voltages, 25 Ω, 16.7 Ω, and 5 Ωloads were used. For other testing, only 16.7-Ω nominal loads were used,corresponding to a 50-Ω load for a 3-capacitor system. Dischargeenergies were determined for full discharge and for discharge to 25% ofcharge voltage. Tests were performed at 37 degree Celsius (°C.).

Capacitance is displayed in FIG. 8. Capacitor 10 made with either Type 1or Type 2 powder but with the same electrolyte 17 have similar ESR. TheType 2 capacitors are lower capacitance, but their ability to be chargedto a higher voltage provides a similar total available energy.

FIG. 9 relates delivered energy for discharge into a 16.7 ohm load to25% of voltage for capacitors made with Type 2 powders. Input anddischarge energies are combined in FIG. 10 as overall efficiency,defined as discharge energy divided by input energy. The figureindicates two runs (“D” and “U”) with indicated electrolyte voltages.Full discharge efficiency starts at about 90% for Type 2 powdercapacitors charged to 150 V. It falls off with applied voltage andhigher nominal electrolyte voltage.

Similar trends are seen in FIG. 11 for discharge to 25% of appliedvoltage. This figure also indicates two runs (“D” and “U”) withindicated electrolyte voltages. Initial efficiencies are slightly lower,in part due to 1/16of the energy remaining on the capacitors; idealefficiency would be only 94%. Inefficiencies increase as applied voltageand electrolyte resistivity are increased.

FIG. 12 shows the marginal efficiency loss of operating at 25% dischargeinstead of at full discharge of capacitor 10. This representative datais taken at 150 V. The dotted line indicates the efficiency lossexpected from the energy remaining on the capacitor. The loss above thisthreshold is related to losses due to slow access to the slug interiorcompared to the time scale of the discharge. These losses are less than2 % for this set of capacitors.

An implantable cardioverter-defibrillator (ICD) application involvescapacitor operation at a very stable temperature (e.g. about 37° C.)with a small number of cycles (e.g. less than 200) and a very short time(e.g. less than 15 minutes) at a certain voltage over the capacitor 10lifetime. These conditions, prevent a higher level of parasitic activityfrom significantly impacting the capacitor lifetime. Additionally, theelectrolyte allows creation of a higher energy density capacitor whichin turn allows for production of a smaller implantable medical device(IMD).

FIG. 13 is a flow diagram for designing an electrolytic capacitor. Atblock 100, at least one capacitor design criteria is considered. Thedesign criteria includes voltage breakdown, ESR, conductivity of anelectrolyte associated with the electrolytic capacitor, and leakagecurrent. At block 110, operating voltage is determined relative toformation voltage. At block 120, an electrolyte is formulated such thatan operating voltage is greater than 90 percent of a formation voltagefor a capacitor.

Skilled artisans will appreciate that alternative embodiments may beformed by implementation of the claimed invention. For example,electrolyte 17 ingredients are selected for certain design conditionsbut different ingredients and/or quantities may be appropriate fordifferent designs. Additionally, adjustment of nominal working voltagemay generally be performed around any desired capacitor operating orsurge voltage.

The description of the invention is merely exemplary in nature and,thus, variations that do not depart from the gist of the invention areintended to be within the scope of the invention. Such variations arenot to be regarded as a departure from the spirit and scope of theinvention.

1. An implantable cardioverter-defibrillator comprising an electrolyticcapacitor comprising: an anode comprising tantalum; a cathode; aseparator coupled to the anode and the cathode; an electrolytecomprising water over the anode, cathode, and separator; and an oxidelayer on the anode that is formed at a formation voltage; wherein,during use of the cardioverter-defibrillator, the capacitor isrepeatedly charged up to 200 times to an operating voltage of greaterthan 90 percent of its formation voltage.
 2. The implantablecardioverter-defibrillator of claim 1, wherein the electrolyte furtherincludes ingredients comprising acetic acid, ammonium acetate,phosphoric acid, and tetraethylene glycol dimethyl ether.
 3. Theimplantable cardioverter-defibrillator of claim 2, wherein the aceticacid includes a molarity which ranges from about 1.5 to about 2.5. 4.The implantable cardioverter-defibrillator of claim 2, wherein theammonium acetate includes a molarity which ranges from about 1.2 toabout 2.1.
 5. The implantable cardioverter-defibrillator of claim 2,wherein the phosphoric acid includes a molarity which ranges from about0.02 to about 0.5.
 6. The implantable cardioverter-defibrillator ofclaim 2, wherein the tetraethylene glycol dimethyl ether includes amolarity which ranges from about 0.8 to about 1.5.
 7. The implantablecardioverter-defibrillator of claim 1, wherein the electrolyte has aworking voltage at least equivalent to its nominal working voltage. 8.The implantable cardioverter-defibrillator of claim 1, wherein theelectrolyte has a working voltage greater than its nominal workingvoltage.
 9. The implantable cardioverter-defibrillator of claim 1,wherein, during use of the cardioverter-defibrillator, the capacitor isrepeatedly charged up to 200 times to an operating voltage of greaterthan 93 percent of its formation voltage.
 10. The implantablecardioverter-defibrillator of claim 9, wherein, during use of thecardioverter-defibrillator, the capacitor is repeatedly charged up to200 times to an operating voltage of greater than 95 percent of itsformation voltage.
 11. The implantable cardioverter-defibrillator ofclaim 10, wherein, during use of the cardioverter-defibrillator, thecapacitor is repeatedly charged up to 200 times to an operating voltageof greater than 98 percent of its formation voltage.
 12. A method forforming an implantable cardioverter-defibrillator comprising anelectrolytic capacitor, the method comprising: selecting an anodecomprising tantalum; coupling a cathode and a separator to the anode;formulating an electrolyte comprising water such that the capacitor hasan operating voltage greater than 90 percent of its formation voltage;placing the electrolyte over the anode, cathode, and separator; andforming an oxide layer on the anode at a formation voltage; wherein,during use of the cardioverter-defibrillator, the capacitor isrepeatedly charged up to 200 times to an operating voltage of greaterthan 90 percent of its formation voltage.
 13. The method of claim 12,wherein formulating an electrolyte comprises selecting ingredientscomprising water, acetic acid, ammonium acetate, phosphoric acid, andtetraethylene glycol dimethyl ether.
 14. The method of claim 13, whereinthe acetic acid includes a molarity which ranges from about 1.5 to about2.5.
 15. The method of claim 14, wherein the ammonium acetate includes amolarity which ranges from about 1.2 to about 2.1.
 16. The method ofclaim 12, wherein the phosphoric acid includes a molarity which rangesfrom about 0.02 to about 0.5.
 17. The method of claim 12, wherein thetetraethylene glycol dimethyl ether includes a molarity which rangesfrom about 0.8 to about 1.5.
 18. The method of claim 12, whereinformulating the electrolyte comprises formulating the electrolyte tohave a working voltage at least equivalent to its nominal workingvoltage.
 19. The method of claim 18, wherein formulating the electrolytecomprises formulating the electrolyte to have a working voltage greaterthan its nominal working voltage.